Many large pipelines are made of reinforced concrete. Such pipelines are used, for example, to move fluids such as potable water or sewage, or to take fluids across some obstacle, such as a river or embankment, by means of an inverted syphon.
In many cases, the concrete pipes used to make the pipelines are prestressed by wrapping tensioned wire around the pipe. Usually this is done by wrapping a single long wire repeatedly around the pipe, often to several thicknesses, and covering it with a grout or concrete layer. Other arrangements such as multiple wires embedded in the concrete are also known. The inclusion of such tensioned wire in the concrete is particularly important to prevent pipeline rupture when the fluid which passes through the pipeline is under pressure.
As pipes age, the wires often break. It has been known that the breakage of tensioning cables or wires in concrete structures causes a noise. Thus, acoustic monitoring of structures can indicate that a tensioning cable or wire has snapped. The breaking of a wire will hereinafter be called "wire breakage".
Attempts have been made to monitor pipelines for an acoustic signal generated by wire breakage by drilling holes through the outer surface of the pipeline wall and mounting hydrophones through the holes. Also, it has been tried to mount a hydrophone on an apparatus suspended in the fluid of the pipeline to listen for such acoustic signal generated by wire breakage. It has also been tried to use an instrumented pig to search for acoustic anomalies. However, these methods are very expensive to implement, and the pig method is not suitable for long term monitoring, as it occludes fluid flow.
Other occurrences of interest in a pipeline can also have acoustic effects. For example, a leak can create acoustic signals. It would be useful to collect the signals from such effects.